What are the procedures and costs for the first extension of stay on a Special Tourist Visa in Thailand?

Timeframe to get a 90 day extension from an STV: Depends on your local immigration office. It's either 45 days or 30 days.

"Chiang Mai, Bangkok, Buriram, Phuket, Chiang Rai and a couple of others all allow people to apply for extensions when they have 45 days or less left on their current stamp. The other immigration offices are 30 days. However, ask your local immigration office for specific details. "

>And is it also the regular 1,900฿ ?

Correct!

I'm also attaching the official requirements from Bangkok Immigration. Your immigration office may be different.